1. Field of Invention
The present invention relates to touch sensor circuit. More particularly, the present invention relates to touch sensor circuit with multiple current sources.
2. Description of Related Art
FIG. 1 is a drawing, schematically illustrating a conventional touch sensor circuit 100. Referring to FIG. 1, a touch sensor circuit 100 includes a pulse signal generator 110, a touch sensing capacitor (CTA), a reset switch (SW1), a charging switch (SW2), a charging capacitor (C1) and an analog-to-digital converter (ADC) 120.
In detail, a first terminal of the pulse signal generator 110 receives a ground voltage GND, a second terminal of the pulse signal generator 110 is coupled to a first terminal of the touch sensing capacitor CTA. The charging switch SW2 is coupled in series between a second terminal of the touch sensing capacitor CTA and an input terminal of the ADC 120. The reset switch SW1 is coupled in series between the second terminal of the touch sensing capacitor CTA and the ground voltage GND.
In operation, when the touch sensor circuit 100 senses a touch event, such as an event that finger of user touches on the touch sensor circuit 100, the touch sensing capacitor CTA responds to the touch event. When a pulse signal PLS issued by the pulse signal generator 110 is applied on the touch sensing capacitor CTA and a pulse signal PLS with different pulse amplitude from the pulse amplitude of the pulse signal PLS before the touch sensing capacitor CTA being touched is produced and transmitted to the charging switch SW2. At this moment, the charging switch SW2 receives the sampling signal SP in a fixed frequency at the control terminal and is accordingly conducted or not-conducted. Here, the pulse signal PLS after sampling the charging is transmitted by the charging switch SW2 to the charging capacitor C1 and charges the charging capacitor C1 and then a detecting voltage Vdet is produced at the input terminal of the ADC 120. After then, the ADC 120 fetches the voltage value of the detecting voltage Vdet. A difference of the voltage values before touching and after touching can be used to judge whether or not at a touching state, that is, whether or not the touch sensor circuit 100 has sensed the touch event. In addition, the reset switch SW1 resets the detecting voltage Vdet according to the reset signal RST.